1. Field of the Invention
The present invention relates to a hydraulic control apparatus of a vehicular automatic transmission and, more particularly, to a fail-safe apparatus that avoids the occurrence of interlock of an automatic transmission.
2. Description of the Related Art
Vehicles provided with an automatic transmission that includes a plurality of friction engagement devices belonging to a first group, and a plurality of friction engagement devices belonging to a second group, and that establishes a plurality of speed change stages of different speed change ratios by simultaneous engagements of one of the friction engagement devices belonging to the first group and of one of the friction engagement device belonging to the second group, are well known. For example, Japanese Patent Application Publication No. JP-A-2000-249219 describes such a vehicle. Japanese Patent Application Publication No. JP-A-2000-249219 discloses a control apparatus constructed as follows. That is, at the time of a failure where two friction engagement devices of the second group simultaneously engage, the control apparatus stops the supply of oil pressure to the friction engagement devices other than the two friction engagement devices used for establishing the predetermined speed change stage through the use of a fail-safe valve, and thereby prevents the interlock of the automatic transmission.
It is conceivable that the shift from the speed change stage maintained immediately prior to the occurrence of a failure to the predetermined speed change stage established at the time of the occurrence of the failure is a downshift. On that occasion, an automatic transmission having multiple speed change stages with an increased number of friction engagement devices may sometimes undergo a downshift of two or more stages from the speed change stage maintained immediately prior to the occurrence of the failure, in which case there is a possibility of occurrence of an uncomfortable behavior of the vehicle.
For example, in an automatic transmission that establishes a plurality of speed change stages of different speed change ratios as indicated in an engagement operation table of FIG. 2, the interlock of the automatic transmission occurs due to simultaneous engagements of any two of friction engagement devices, that is, a clutch C3, a clutch C4, and a brake B1. Therefore, in this automatic transmission, it is conceivable to provide a fail-safe valve so as to prevent simultaneous engagements of those friction engagement devices. Let it assumed that the clutch C3, the clutch C4 and the brake B1 are given degrees of priority in the order of, for example, clutch C3, the clutch C4 and the brake B1, in order to engage one of them with priority to establish a speed change stage. Then, at the time of a failure where the oil pressure for engagement of the clutch C4 is output during running with the eighth speed change stage, the clutch C4 is engaged with priority over the brake B1, to establish the sixth speed change stage, thus bringing about a downshift of two or more stages, which is likely to lead to an uncomfortable behavior of the vehicle. On the assumption that degrees of engagement priority are given in the order of the clutch C3, the brake B1 and the clutch C4 in order to avoid the occurrence of a downshift of two or more stages during running with the eighth speed change stage, if during running with the fourth speed change stage, a failure occurs where the oil pressure for engagement of the brake B1 is output, the brake B1 is engaged with priority over the clutch C4, thus bringing about a downshift of two or more stages, which is likely to lead to an uncomfortable behavior of the vehicle.